A control valve and its operation are known in the art and there is no need to describe it in more detail in this connection. The valve can be a quarter-rotational or a linear valve, said designations describing the moving direction of the closing element of the valve in the controlling situation. A quarter-rotational valve can be e.g. a ball valve or a butterfly valve. Examples of a ball valve have been described e.g. in U.S. Pat. No. 4,747,578. The valve is operated by means of an actuator that turns the turning shaft of the closing element between the closed and the opened position. The actuator can be driven by means of a cylinder piston device controlled by a pilot valve. This pilot valve is located in the positioner of the control valve. The positioner is a device for amplifying the control signal into the operation pressure of the pneumatic actuator. In the electropneumatic positioner, the electric signal is amplified into the pneumatic operation pressure. The positioner, by means of a feed-back element, also positions the valve to correspond to the control signal.
The diagnostics of control valves equipped with a pneumatic actuator is in general based on tests performed on the control valves, such as step-function response and hysteresis tests. A requirement for performing the tests is that the process is stopped, whereby the tests are performed at site of the valves. The testing equipment often also includes complicated sensors that can make the testing on the field complicated (U.S. Pat. No. 5,197,328). This is so called off-line diagnostics.
The diagnostics can also be performed on-line, whereby the positioner of the control valve monitors e.g. the position message of the valve and gives an alert if the position message deviates too much from the value presupposed by the control system. Part of the on-line diagnostics are also the operation counters of the valve that give an alert when the number of the operations exceeds a certain predetermined limit. This diagnostics system, however, does not include any deduction procedure by which the faults could be located. In both of the U.S. Pat. No. 5,329,465 and U.S. Pat. No. 4,694,390, valves have been described, in the control system of which separate measuring sensors are used. The inventions described in the publications concern an on-off regulation of the valve, not continuous controlling.
In the system according to U.S. Pat. No. 5,329,465, sensors are used for condition surveying of the valve and for collecting historical data for the data base. The fault analysis is made by comparing the measured values with the values collected earlier for the data base. The position of the valve is not adjusted continuously but the valve is moving only when it is closed or opened. This can happen a couple of times per day or the adjustment interval can even be several months. The values required for the fault analysis are monitored only during this opening and closing movement that happens at long intervals.